Higher oil prices and more stringent environmental regulations have been the impetus for allocating greater resources into research and development for alternate and renewable fuels worldwide. An important development in this regard is the use of ethanol as the blending stock for gasoline. Burning ethanol instead of gasoline could reduce carbon emissions by more than 80% and completely eliminate the release of acid-rain-causing sulfur dioxide. The U.S. Department of Energy (DOE) predicts that ethanol could reduce gasoline consumption by 30% in the United Sates by 2030. A recent DOE report concluded that, in terms of key energy and environmental benefits, fermentation ethanol is clearly superior to petroleum-based fuels, and future cellulosic-based ethanol will be even better.
A major drawback of using ethanol is that fermentation yields dilute aqueous ethanol mixtures that contain only 10-12 wt % ethanol. Current dehydration strategies, for producing anhydrous (99.5 wt %) ethanol that is suitable for gasoline blending, are energy intensive and employ conventional distillation and a subsequent step to break the ethanol-water azeotrope. As compared to the energy value of ethanol which is 21,400 KJ/L, conventional distillation requires approximately 6,400 KJ/L to concentrate fermentation ethanol to 95.6 wt % ethanol, which is the azeotropic composition. Even improved distillation schemes that employ heat integration still require 5,500 KJ/L to produce the azeotropic composition. Thereafter, concentrating the ethanol from the azeotropic point to produce anhydrous ethanol requires more energy. As is apparent, despite the benefits of using ethanol, the energy costs associated with dehydrating ethanol present serious economic impediments for using ethanol produced by fermentation as a gasoline blending stock or as an engine fuel.
The major commercial methods for concentrating ethanol beyond the azeotropic composition for producing anhydrous ethanol are: (1) azeotropic distillation, (2) molecular sieves adsorption, and (3) extractive distillation. A fourth method known as membrane separation which uses zeolitic or polymer membranes to break the azeotrope is largely in the developmental stages.
In azeotropic distillation, a volatile entrainer is added to an aqueous ethanol feed mixture. The entrainer modifies the activity coefficients of the water and ethanol being separated and forms an azeotrope with the water to be taken overhead as the process yields anhydrous ethanol. Pentane, benzene, diethyl ether, and gasoline have been disclosed as suitable entrainers. See, U.S. Pat. No. 3,575,818 to West, U.S. Pat. No. 2,012,199 to McElroy, U.S. Pat. No. 2,371,010 to Wolfner, and Black et al., “Extractive and Azeotropic Distillation,” Am. Chem. Soc. Advances In Chemistry Series No. 115, p. 64, 1972. The major disadvantage of the azeotropic method is that the ethanol feed has to be pre-concentrated to near 95 wt %, the azeotropic composition, which is an energy intensive process in itself.
Adsorptive separation processes for separating of ethanol from water are typically batch processes, with respect to the adsorbents used, that entail an adsorption cycle and a separate desorption cycle. Various adsorptive separation techniques are described in, for example, U.S. Pat. No. 4,273,621 to Fornoff, U.S. Pat. No. 4,407,662 to Ginder, U.S. Pat. No. 4,465,875 to Greenbank et al., U.S. Pat. No. 4,287,089 to Convers et al., U.S. Pat. No. 4,277,635 to Oulman, U.S. Pat. No. 4,382,001 to Kulprathipanja et al., U.S. Pat. No. 5,030,775 to Sircar, U.S. Pat. No. 4,343,623 to Kulprathipanja et al., U.S. Pat. No. 4,319,058 to Kulprathipanja et al, U.S. Pat. No. 5,766,895 to Valkanas et al, U.S. Pat. No. 4,359,593 to Feldman and U.S. Pat. No. 2,137,605 to Derr. Although adsorption methods are very selective in removing water or ethanol, the associated high heat energy requirements, high operating costs, limited capacities, and uncertainty in the lengths of adsorbent lives are major drawbacks toward commercial operations.
Finally, in extractive distillation (ED) a high-boiling, polar, nonvolatile solvent is added to the upper portion of an extractive distillation column (EDC), while the feed containing ethanol and water is fed the middle or lower portion of the EDC, which is below the solvent entry point. Depending upon the properties of the solvent, the descending nonvolatile solvent preferentially extracts water or ethanol from the ascending vapor stream thereby eliminating the ethanol-water azeotrope and producing purified ethanol or water from the overhead of EDC. A portion of the EDC overhead stream is recycled to the top of the EDC as reflux. Saturated solvent that is rich in water or ethanol is then withdrawn from the bottom of EDC and transferred to the middle portion of a solvent recovery column (SRC). Water or ethanol in the saturated solvent is stripped from the solvent by heat from a SRC reboiler and then recovered from the overhead stream of the SRC as purified water or ethanol. Again, a portion of the overhead stream is recycled to the top of SRC as liquid reflux. Lean solvent from the bottom of SRC is circulated back to the EDC as the solvent feed.
An ED process for dehydrating aqueous ethanol using glycerin as the ED solvent was disclosed in U.S. Pat. No. 1,469,447 to Schneible. Subsequently, other ED solvents considered include: ethoxyethanol and butoxyethanol (U.S. Pat. No. 2,559,519 to Smith et al.), butyl, amyl or hexyl alcohols (U.S. Pat. No. 2,591,671 to Catterall), gasoline components (U.S. Pat. No. 2,591,672 to Catterall), sulfuric acid, acetone or furfural (U.S. Pat. No. 2,901,404 to Kirshenbaum et al.), 2-phenyl phenol, or mixtures of 2-phenyl phenol and cumyl phenol (U.S. Pat. No. 4,428,798 Zudkevitch et al.), cyclohexylcyclohexanone or cyclohexylcyclohexanol (U.S. Pat. No. 4,455,198 to Zudkevitch et al.), methyl benzoate, mixture of methyl benzoate and trimellitic anhydride, and mixture of dipropylene glycol dibenzoate, ethyl salicylate and resorcinaol (U.S. Pat. No. 4,631,115 to Berg et al.), hexahydrophthalic anhydride, mixture of methyl tetrahydrophthalic anhydride and pentanol-1, and mixture of trimellitic anhydride, ethyl salicylate and resorcinol (U.S. Pat. No. 4,654,123 to Berg et al.); and diaminobutane, 1,3-diaminopentane, diethylenetriamine, and hexachlorobutadiene (U.S. Pat. No. 6,375,807 to Nieuwoudt). The ED solvents disclosed in these patents were said to be particularly selective in separating ethanol and water, but without regard to their practical applicability to ED processes with respect to other important solvent properties such as solvent thermal stability, toxicity, boiling point, etc. These patents also did not address energy and related economics issues related to ED processes.
In order to reduce the energy requirements in ED processes, U.S. Pat. No. 4,400,241 to Braithwaite et al. proposed adding alkali-metal or alkaline-earth metal salts to a polyhydric alcohol solvent to enhance the ED solvent performance. The preferred systems include (1) sodium tetraborate that is added to ethylene glycol and (2) dipotassium phosphate that is added glycerin.
Other approaches to reducing energy requirements featured improved process designs to recovery energy such as that in U.S. Pat. No. 4,349,416 to Brandt et al. where a first side stream is withdrawn from the EDC, passed in heat exchange with the bottoms from the EDC en route to the SRC and returned to the EDC at a point below the point of the side stream. A second side stream from the EDC is also withdrawn, passed in heat exchange with the bottoms of the SRC and returned to the EDC.
U.S. Pat. No. 4,559,109 to Lee et al. disclosed another approach whereby aqueous ethanol containing 10-12 wt % ethanol is first converted to an 85-90 wt % concentrated vapor in a front-end distillation column which is then fed to an EDC. The front-end distillation column represents the water-rich (lower) portion of the fractionator, while the EDC represents the ethanol-rich (upper) portion of the fractionator. Extractive solvent is added only to the EDC (ethanol-rich portion of the fractionator) where the vapor-liquid equilibrium (VLE) curve is very unfavorable for distillation. The solvent is said to eliminate the binary ethanol-water azeotrope and modify the shape of the ethanol-rich portion of the VLE curves favorably for distillation. The ethanol-water saturated solvent is then completely removed via a rich solvent bottoms stream from the EDC and fed to a SRC (solvent stripper). The vaporous overhead stream of the SRC is said to be recycled directly to an upper portion of the front-end distillation column while lean solvent from the bottom of the SRC is fed to the top of the EDC. In a preferred application of the process, the rich solvent bottoms stream from the EDC which is fed to the SRC contains more than 40 wt % of the ethanol that is present in the vaporous feed to the EDC from the front-end distillation column (or simply more than 40 wt % of the ethanol in the feed to the front-end distillation column).
In practice the techniques described in U.S. Pat. No. 4,559,109 are neither energy nor cost effective because the rich solvent stream, which contains 40 wt % of the feed ethanol, must be vaporized in the SRC and then again in the pre-distillation column. Initially the rich solvent is vaporized in the SRC to separate the solvent from the water and the ethanol, but it has been demonstrated that the water and the ethanol cannot be directly recycled to the pre-distillation column in the form of vapor as stated in the patent. The reason is that the SRC is normally operated under reduced pressures in order to lower the bottoms temperature so as to minimize solvent decomposition. When ethylene glycol is employed as the ED solvent, as shown in FIGS. 5 and 6 of the patent, the SRC should be operated at substantially lowered pressures of about 150 mmHg at the top in order to keep the bottoms temperature below 180° C. The pre-distillation column, on the other hand, is operated at higher pressures than that of the EDC to allow the vapor stream to be fed from the overhead of the pre-distillation column to the EDC, which is preferred procedure as described in the patent. Because of these constraints, the overhead vapor stream of the SRC has to be condensed before it can be pumped into the higher pressured pre-distillation column, where the ethanol content in the condensed SRC recycle stream is re-vaporized. Another problem associated with the process depicted in U.S. Pat. No. 4,559,109 is that there is no liquid reflux for the SRC which causes the upper trays in the SCR to run dry.
Other energy saving ED techniques employ a combination of the basic distillation method along with (1) multi-effect distillation, (2) conventional overhead-to-reboiler heat pumped distillation, (3) azeotropic and extractive distillation, or (4) fermentative production of volatile compounds, which is disclosed in U.S. Pat. No. 4,961,826 to Grethlein et al. All of the energy saving methods in the prior art for the ED process or combination process schemes associated ED methods are either too limited in scope for energy savings or too complicated for practical applications.